Non-positive system used until February 1964. This uses a hose from the rocker cover to the front air-cleaner (outside the filter) and a non-ventilated oil filler cap. (Photo: Clausager, Original MGB)

There is also a hose on the front tappet chest cover clipped to a stud or blanking-plate screw where the mechanical fuel pump is on other models, just hanging down with an open end. (Image: Auto-Part.com)

In February 1964 a positive ventilation system was implemented. Until October 1968 it consisted of a PCV valve mounted on the inlet manifold, plumbed to a port on the front tappet chest cover. The oil filler cap is now ventilated with a restriction and a filter. (Photo: Clausager, Original MGB)

Cover 12H1399 (above) with internal filtering and oil separation, used with the PCV valve. (Chris Howells)

In October 1968 the PCV valve was replaced by carb ventilation, with ports on both carbs connected together and taken down to the front tappet chest cover. The oil filler cap is ventilated as before. This system was used on UK cars until the end of production.

Cover with external filter and separator 12H3684 used with carburettor ventilation until the 18V779/780 engines for the 74 model year. However the Parts Catalogue states that rebuilt engines used the cover below, as is the case on Bee's Gold Seal engine. Image from William Revit in Tasmania via the MG Enthusiasts forum.

Cover 12H4395 (below) again with internal filtering and oil separation, used with original 18V797 and later engines, and for all rebuilt 18GG and later engines. (Chris Howells)

North American spec from October 1969 with the charcoal canister. Originally with twin SUs connected to the front tappet chest cover as before, but with a non-vented oil-filler cap. This version shows the later single Zenith/Stromberg carb and anti-runon valve. (Photo: Clausager, Original MGB)

V8 with individual hoses and oil/flame traps from carbs to rocker covers.

Rear of the V8 air-cleaner box showing the breather intake filter with mounting clip and hoses.